This invention relates to scroll compressors, and in particular to a scroll compressor with increased efficiencies. Although compressors are used for example, this invention applies to scroll vacuum pumps and air motors equally.
Scroll compressors are often used in equipment such as oxygen concentrators and refrigerators. Scroll compressors are preferred for such applications because they tend to be quieter in operation than reciprocating compressors. Scroll compressors include two involutes or wraps which are meshed and define suction areas or zones at their outer edges. Fluid voids are defined by the two involutes between their points of contact. One involute is fixed and the other is orbited, by an electric motor, for example. The orbiting motion of the orbiting involute causes the fluid voids to move toward the center of the involutes and become smaller to compress the fluid contained therein. The outlet is at the center of the scroll and the compressed fluid is released at that point.
The involutes are maintained in a specific phase relationship. For the compressor to operate properly, the phase relationship between the two involutes must be maintained. Typically, oldham couplings have been used to maintain the phase relationship. However, these couplings require lubrication. If there is insufficient lubrication in the coupling, the compressor will fail. Others have used idler cranks to maintain the phase relationship. Such systems are shown, for example, in U.S. Pat. Nos. 4,192,152 to Armstrong et al and 5,154,592 to Ohtani et al. Both these compressors place idler cranks at the periphery of the scrolls. The idler cranks maintain the two scrolls in the proper phase relationship. However, they do not allow for harnessing of the rotary motion of the crank. This motion could be used to drive other items, such as fans.
The running clearance between the fixed and orbiting scroll members must be precisely controlled for the compressor to operate properly. Hard machined stops in either the housing or fixed scroll have been used to control the running clearance. However, a hard stop is not suitable for non-lubricated compressors. The running clearance has also been controlled using precision angular contacts or spherical roller bearings. U.S. Pat. No. 4,472,120, to McCullough, is one example of a compressor using spherical roller bearings. These bearings, however, are very expensive.
The running clearance between the fixed and orbiting scroll members creates a "blow hole" formed by the space between the tip of one involute and the plate of the opposing scroll member. This "blow hole" creates leaks in the fluid pockets which decreases the compressors performance. It is thus important that the seal between a wrap tip and the base of its opposing scroll be maintained as tight as possible. Maintaining the running clearance between the wrap tip and the opposing scroll base is complicated by the heat generated during operation of the compressor. Heat generation is not constant along the length of the scroll. More heat is generated at the center of the scroll, near the outlet, than at the beginning of the scrolls, near the inlet or suction areas. Some compressors have used compliance seals to maintain the blow hole closed while at the same time allowing for expansion of the involute along its length. Other compressors, such as the Ohtani et al compressor, do not use compliance seals. Rather, they change the height of the scroll along its length to accommodate the expansion of the scroll during operation. This of course will not maintain the blow hole closed at all times thus adversely affecting the compressor's performance. To avoid the use of compliance seals, a great deal of precision must be incorporated into the manufacture of the components parts. It becomes necessary to precisely maintain the relationship of the compressor housing with the fixed scroll and the central bearing within the housing. The central drive bearing in the orbiting scroll must also be precisely located. All this precision greatly increases the cost of the compressor.
Heat generation can, of course, be minimized by efficient heat dissipation. Ribs have been used to dissipate heat and to strengthen the scrolls. Typically, these ribs extend radially along an outboard surface of the scrolls. The ribs also serve to make the scrolls rigid to minimize deflection and distortion. Rigid scrolls aid in optimizing scroll performance. The position and formation of the ribs can be improved upon to both strengthen the scrolls and to improve heat dissipation.
It is often desirable to vary the displacement of a scroll compressor by a relatively small mount to allow for customer variations or motor frequency variation of 50 or 60 Hz, for example. In the past, this has been done in one of two ways. One method of varying the compressor displacement was to vary the height of the scroll involute. However, varying the height of the scroll involute requires changes to the idler cranks, counterweights, and housing to accommodate the change in the mass of the orbiting scroll and the change in the involute height. The second method was to shorten the involute wraps. This will reduce the compressor displacement without having to change the idler cranks and housing, however, the orbiting scroll mass is still changed and the counterweights must be adjusted accordingly. Also, shortening the involute wraps will effect compressor efficiency.